Study On The Technology For Preparing Lithium Iron (?) Phosphate From Lithium Iron (?) Phosphate Hydroxide Produced By Hydrothermal Method

The lithium iron phosphate?LiFePO₄?with olivine structure has been widely used as the cathode active material in lithium ion battery,which serves as the power of electronic vehicle and energy reservoir.There are shortcomings for LiFePO₄,they are its low intrinsic electron conductivity and small electro-migration coefficient of lithium ion.Several methods are used to overcome these shortcomings,such as coating carbon to improve its electronic conductivity,reducing its particle size to short the path of lithium ion diffusion and electron conduction,and making lattice defect by introducing outside metal ion to improve lithium ion diffusion.On the base of searching for the data of JCPDS there are 123 different compounds which only contain Fe,P,O,H,these compounds contain FePO₄·2H₂O,while the molar ratio between iron to phosphorus and the number of crystal water in these compounds were differ far way compared with FePO₄·2H₂O.So there is a difficulty while using FePO₄·2H₂O.as raw material in the technology for preparing pure olivine phase LiFePO₄ industrially,because the molar ratio of iron to phosphorus to lithium in the raw material or precursor must be one to one to one.On the base of searching for the data of JCPDS there are only two kinds of compounds which contain Li,Fe,P,O,H,the one is acid lithium iron pyrophosphate LiFeH₄?P₂O₇?₂ and the other is lithium iron phosphate hydroxide LiFePO₄?OH?.In this paper we try to exploring the possibility to use LiFePO₄?OH?as raw material or precursor to synthesis pure olivine LiFePO₄.The optimum parameters of the technology of carbon thermal reduction method for producing LiFePO₄ and of hydrothermal method for producing LiFePO₄?OH?were researched systematically.The main results are shown as follow:?1?The preparation technology feasibility of olivine structured LiFePO₄ with LiFePO₄?OH?precursor as raw material by carbon thermal reduction was studied.The effect of carbon thermal reduction temperature,carbon thermal reduction time and carbon coating contents on the particle diameter,particle morphology and electrochemical performances of LiFePO₄/C composites was discussed.The results showed that: when the carbon thermal reduction solid reacted at 650 °C for 8 h under carbon coating content of 3.21 wt.%,the as-prepared LiFePO₄/C composites exhibited a good electrochemical performance.The composites delivered a initial discharge specific capacity of 140.72 mAh/g at 0.1 C;the capacity always remained at 93.97% after 100-times' cycling in the 1.0 C.?2?This paper also investigated the effect of hydrothermal temperature and hydrothermal time on the hydrothermal products LiFePO₄?OH?precursors and the corresponding final products—LiFePO₄/C composites.The optimized process parameters showed that: the particle size of the LiFePO₄/C composites prepared under hydrothermal temperature of 180 °C and hydrothermal time of 8 h were 200-300 nm,which were the high pure phase crystallinity.The LiFePO₄/C composites synthesized by the corresponding LiFePO₄?OH?precursor showed the best performance,the initial discharge specific capacity was 153.32 mAh/g at 0.1 C.?3?The Mg doping LiFePO₄?OH?precursor was prepared during the course of the hydrothermal reaction.On the basis of the optimization of hydrothermal reaction condition?180 °C,48 h?,this paper discussed the effect of Mg doping on hydrothermal products LiFePO₄?OH?precursors and the corresponding LiFePO₄/C composites.The results showed that: compared with Mg undoping LiFePO₄?OH?precursor,Mg doping LiFePO₄?OH?precursor secondary particle was spherical particle of about 5 ?m,its primary particle was cuboid block structure of 300-500 nm,the secondary particle was packed close by the primary particle.The LiFePO₄/C composites prepared by spherical Mg doping LiFePO₄?OH?precursor didn't show this spherical structure,but its electrochemical properties improved.When the Mg doping contents of LiFePO₄?OH?precursor was 1 mol%,the discharge specific capacity of as-prepared LiFePO₄/C composites reached 160.92 mAh/g at 0.1 C.